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Rocky
Mountain Region 2010 Fall Color Report

Every fall across the Northern Hemisphere, diminishing daylight
hours and falling temperatures induce trees to prepare for winter
and they shed billions of tons of leaves. In the Rocky Mountain
Region, a spectacular color show precedes the shedding of leaves.
Formerly green leaves turn to brilliant shades of yellow, orange,
and red as a result of transformations in leaf pigments.

The green pigment in leaves is chlorophyll, which
absorbs red and blue light from sunlight.
Therefore, the light the leaves reflect is diminished in red
and blue and appears green. The molecules of chlorophyll are large
(C55H70MgN4O6). They
are not soluble in the aqueous solution that fills plant cells.
Instead, they are attached to the membranes of disc-like structures,
called chloroplasts, inside the cells. Chloroplasts are the site
of photosynthesis, the process in which light energy is converted
to chemical energy. In chloroplasts, the light absorbed by chlorophyll
supplies the energy used by plants to transform carbon dioxide and
water into oxygen and carbohydrates, which have a general formula
of Cx(H2O)y.

x CO2 + y H2O

light

x O2 + Cx(H2O)y

chlorophyll

In this endothermic transformation, the energy of
the light absorbed by chlorophyll is converted into chemical energy
stored in carbohydrates (sugars and starches). This chemical energy
drives the biochemical reactions that cause plants to grow, flower,
and produce seed.

Chlorophyll is not a very stable compound; bright
sunlight causes it to decompose. To maintain the amount of chlorophyll
in their leaves, plants continuously synthesize it. The synthesis
of chlorophyll in plants requires sunlight and warm temperatures.
Therefore, during summer chlorophyll is continuously broken down
and regenerated in the leaves of trees.

Another pigment found in the leaves of many plants
is carotene. Carotene absorbs blue-green and blue light. The light
reflected from carotene appears yellow. Carotene is also a large
molecule (C40H36) contained in the chloroplasts
of many plants. When carotene and chlorophyll occur in the same
leaf, together they remove red, blue-green, and blue light from
sunlight that falls on the leaf. The light reflected by the leaf
appears green. Carotene functions as an accessory absorber. The
energy of the light absorbed by carotene is transferred to chlorophyll,
which uses the energy in photosynthesis. Carotene is a much more
stable compound than chlorophyll. Carotene persists in leaves even
when chlorophyll has disappeared. When chlorophyll disappears from
a leaf, the remaining carotene causes the leaf to appear yellow.

The prime example is the early season color change showing the unique characteristic of aspen growth.Â Aspen propagate
primarily by sprouting from an expanding root system, creating
groups of trees, or clones, ranging in size from several trees
to many acres. These clones are genetically identical.Â One factor
in how and when a tree changes color is the balance of various
chemicals in the plant.Â This balance is, to some degree, genetically
determined and varies between clones.Â Because of these differences,
it is possible to see hillsides with one small group of trees that
have already changed to vibrant gold colors standing among otherwise
green aspen.Â This color change allows viewers to readily see individual
aspen clones.

A
third pigment, or class of pigments, that occur in leaves are the
anthocyanins. Anthocyanins absorb blue, blue-green, and green light.
Therefore, the light reflected by leaves containing anthocyanins
appears red. Unlike chlorophyll and carotene, anthocyanins are not
attached to cell membranes, but are dissolved in the cell sap. The
color produced by these pigments is sensitive to the pH of the cell
sap. If the sap is quite acidic, the pigments impart a bright red
color; if the sap is less acidic, its color is more purple. Anthocyanin
pigments are responsible for the red skin of ripe apples and the
purple of ripe grapes. A reaction between
sugars and certain proteins in cell sap forms anthocyanins. This reaction does not
occur until the sugar concentration in the sap is quite high.
The reaction also requires light, which is why apples often appear
red on one side and green on the other; the red side was in the
sun and the green side was in shade.

During summer, the tree leaves are factories producing
sugar from carbon dioxide and water using by the action of light
on chlorophyll. Chlorophyll causes the leaves to appear green. (The
leaves of some trees, such as birches and cottonwoods, also contain
carotene; these leaves appear brighter green, because carotene absorbs
blue-green light.) Water and nutrients flow from the roots, through
the branches, and into the leaves. Photosynthesis produces sugars
that flow from the leaves to other tree parts where some of the
chemical energy is used for growth and some is stored. The shortening
days and cool nights of fall trigger changes in the tree. One of
these changes is the growth of a corky membrane between the branch
and the leaf stem. This membrane interferes with the flow of nutrients
into the leaf. Because the nutrient flow is interrupted, the chlorophyll
production in the leaf declines, and the green leaf color fades.
If the leaf contains carotene, as do the leaves of birch and hickory,
it will change from green to bright yellow as the chlorophyll disappears.
In some trees, as the sugar concentration in the leaf increases,
the sugar reacts to form anthocyanins. These pigments cause
the yellowing leaves to turn red. Red maples, red oaks, and sumac
produce anthocyanins in abundance and display the brightest reds
and purples in the fall landscape.

The
range and intensity of autumn colors is greatly influenced by the
weather. Low temperatures destroy chlorophyll, and if they stay
above freezing, promote the formation of anthocyanins. Bright sunshine
also destroys chlorophyll and enhances anthocyanin production. Dry
weather, by increasing sugar concentration in sap, also increases
the amount of anthocyanin. So the brightest autumn colors are produced
when dry, sunny days are followed by cool, dry nights.

In recent years, fall colors have been attracting
more travelers to prime color regions: New
England, Michigan, Wisconsin, and Colorado. The right combination of tree species
and likely weather conditions produce the most spectacular displays
in these regions. States in these regions maintain a fall foliage
"hotline," keeping color watchers apprised of the peak viewing locations
and times. The Forest Service also operates a Fall Foliage
Hotline at 800-354-4595. A detailed report can be found on the
Web at www.fs.fed.us/news/fallcolors/.